1. Field of the Invention
The present invention relates to a multi-domed switch of the push-button type for a thin printed circuit and to a thin printed circuit. Such a thin printed circuit may be incorporated into a chip card (also called a smart card).
2. Description of Related Art
Contact members called “domes” are frequently used as elements of a push-button switch in a keyboard. A dome is an element in the form of a spherical cap, the cap either being continuous or peripherally notched, which exerts a force in reaction to a pressure that varies as shown in FIG. 6.
The curves in FIG. 6 show, plotted on the y-axis, the variation in the force applied at the centre of a dome as a function of the travel (plotted on the x-axis) of this central part of the dome. The upper curve corresponds to the increase in pressure applied to the dome, into its contact position, and the lower curve corresponds to the relaxing of this pressure, into the rest state. The upper curve shows that the necessary force increases up to a maximum Ft for a travel of Tt before rapidly decreasing down to a smaller value Fr for a travel Tr before a slight rise in the force until complete contact.
The part of this curve in which the force decreases while the travel between the travels Tt et Tr increases gives a tactile effect felt by the finger exerting a pressure on the dome. The first part of the travel may be as gradual as desired, by applying a force that increases slowly. However, once the maximum force Ft is reached, the transition to straight contact takes place very rapidly through the action of the elasticity of the dome itself.
It is desirable to use such domes in push-button switches incorporated into thin circuits. The advantage of these push-button switches is that they allow the formation of “keyboard” or control keys incorporated into thin members. Thus, to produce relatively flexible chip cards provided with a keyboard and with a display, it is desirable to use contact domes for push-button switches.
It has therefore been attempted to incorporate domes of various shapes into such chip cards. Owing to the dimensional constraints of these cards, the domes must have a diameter of about 5 to 8 mm in the plane of the card and a travel of few tenths of a millimeter, namely less than 0.45 mm. No construction of a domed push-button switch has been satisfactory in this application. The reasons for this are in general either the absence of a discernible tactile reaction or more often insufficient reliability.